Surgical stapling apparatus for stapling tissue are well known in the art and typically include a handle assembly, a body portion extending distally from the handle assembly, and a tool assembly supported on the distal end of the body portion. The tool assembly includes first and second jaws which are movable in relation to each other between unapproximated and approximated positions. The first jaw supports an anvil assembly and the second jaw supports a cartridge assembly which houses a plurality of staples. The cartridge can also include a knife for severing tissue. In known apparatus, a fired or spent cartridge can be replaced with an unfired or fresh cartridge to facilitate reuse of the surgical stapling apparatus.
In order to prevent refiring of the surgical stapling apparatus with a spent cartridge, or to prevent firing of a surgical stapling apparatus which does not include a cartridge, it is known to provide a lockout mechanism which prevents advancement of a drive member of the stapling apparatus. Although known lockout mechanisms are effective to prevent firing of a surgical stapling apparatus which includes a spent cartridge or does not include a cartridge, an improved, reliable lockout mechanism is desired.